1. Field of the Invention
This invention relates to the prevention and/or suppression of discoloration in polyolefins protected by antioxidants as well as the materials and method therefor.
2. Description of the Prior Art
Various polyolefins are commercially available and have found numerous uses. The most commonly used polyolefins for various commercial products are polypropylene, high density polyethylene, low density polyethylene, linear low density polyethylene, and polybutylene. Articles of manufacture made from such polyolefins are made by extrusion molding, injection molding, blow molding, film extrusion and other methods of resin processing. During such processing the polyolefin resins are subjected to elevated tempertures. The elevated temperatures noted here are high enough to affect the rigidity of the polyolefin, but generally are kept below the decomposition point of the polymer.
Some examples of commercial polyolefins include:
(1) polypropylene, such as Profax 6501 (TM, Hercules Powder, Inc.), Dypro (TM, ARCO Chemical Co.); PA1 (2) high density polyethylene, such as Marlex (TM, Phillips Chemical Co.); PA1 (3) low density polyethylene, such as Norchem NPE 420 (TM, Northern Petrochemicals Co.); PA1 (4) linear low density polyethylene (LLDPE), such as Dowlex 2252 (TM, Dow Chemical Co.); LPX-2 and LPX-7 (TM's, Exxon Corp.) and Union Carbide UCC-1055 (TM); and PA1 (5) polybutylene from Witco Chemical Co.
The polyolefins mentioned above are used in making both flexible and rigid plastic products including toys, office materials, containers, home products, cable coatings, film, fibers, and other articles used in industry.
The polyolefins mentioned above ordinarily require antioxidant protection to avoid molecular degradation or other deterioration associated with exposure to air and elevated temperatures. For example, unstabilized Profax 6501 polypropylene "falls" in oven testing at 150.degree. C. after only about 3-4 hours. However, addition to the Profax 6501 polypropylene of any of the various known and commercially used antioxidants extends the life of this polyolefin significantly. For example, the combination of 1,3,5-trimethyl-2,4,6-tris (3',5'-di-tert-butyl-4'-hydroxybenzyl)benzene marketed by Ethyl Corporation as Ethanox.RTM.330 antioxidant with the polypropylene extends the lifetime in oven aging at 150.degree. C. to greater than 1,000 hours.
The history of polyolefin production includes a progression of catalyst systems and post-polymerization recovery techniques which affect the purity of the bulk polyolefin. In the 1960's and 1970's a slurry process such as that of Phillips Petroleum Company was used. The slurry process required a catalyst separation step which, although it was more effective for recovering catalyst than today's methods, was nevertheless quite costly in terms of handling, catalyst losses, and solvent costs for the separation. Such a process is described and compared to newer processes in the Chemical Economics Handbook, SRI International, Plastics and Resins, 580.1342 A, August, 1981. The Encyclopedia of Polymer Technology, Volume II (1981) at page 282 also compares the Phillips' slurry process with the (relatively) new Ziegler process. At page 605, the post-polymerization purification of propylene is discussed. Therein it is noted that residual metals remain in the polymer product.
Another example of the Phillips' slurry technique is U.S. Pat. No. 2,825,721 to Hogan et al (1958). Again in Phillips' U.S. Pat. No. 3,502,634 (1970) to Stedefeder et al, the slurry process is disclosed where 5000 parts methanol wash are used for 1.25 parts titanium trichloride catalyst.
In Phillips' U.S. Pat. No. 3,922,249 (1975) it is disclosed that N,N-bis(hydroxyalkyl)-N-alkylamines used as antistatic agents cause discoloration in polyolefins produced by the slurry method. The amines include, for example, Armour Industrial Chemical Company's Armostat 310, N,N-bis(2-hydroxyethyl)-N-octadecylamine.
Discoloration by the listed amines was known and the invention disclosed in the U.S. Pat. No. 3,922,249 patent was the use of certain phosphites to correct discoloration caused by the antistatic agents.
Most large volume polyolefins including high density polyethylene, low density polyethylene, linear low density polyethylene, polybutylene, polypropylene and the like are now produced by processes using metal-containing catalysts wherein trace amounts of the catalyst metals are not recovered but remain in the polymer.
While all of the effects of catalyst trace metals in polymers are not known, it is known that most polymers produced today, especially those relatively "dirty," high volume polymers for general purpose grade applications tend to discolor with the addition of effective amounts of various types of antioxidants. Of course without the antioxidants, the service life of the polymer product is extremely short.
Color is a significant problem with combinations of polyolefin and antioxidant. While this problem may be viewed as merely an aesthetic one, it is nevertheless significant. Articles of manufacture made from antioxidant-protected polyolefins must in many cases have an initial white or transparent color and maintain such color in order to be suitable for commercialization.
Special problems may be presented for containers and other articles of manufacture wherein a standard white color is necessary for production of a finished product and/or before modification to a different color.
Some of the polyolefin and antioxidant combinations evidence a greater degree of discoloration than others. Furthermore, the degree of initial discoloration and further discoloration with time varies not only according to the amount of antioxidant present, but also with the particular type of polyolefin used, the number of extrusion passes, the severity of processing, and the presence of contaminants.
Various nitrogen-containing materials are known in the prior art for purposes associated with resins and the like. For example, U.S. Pat. No. 3,553,298 to Hodan et al lists various amines, heterocyclic alkyls, and other nitrogen-containing compounds for combination with phosphite esters. The disclosure of the Hodan et al patent is directed toward such a combination for the purpose of stabilizing the phosphite ester against hydrolysis in a manner more advantageous than the materials used theretofore. Such amines have also found use in distillate fuels.